This invention relates to synthetic medical dressings or coverings for wounds and to processing techniques for preparing such dressings or coverings. More particularly, it relates to thin drapable sheets of polymeric material which have physiologic properties similar to human skin plus other desirable properties which made the sheets especially useful in the treatment of thermal wounds, conventionally classified as "burns". The medical problems posed by burn wounds and the general requirements for the successful treatment of such wounds are known to those skilled in the art. See, for example, the discussion in U.S. Pat. No. 3,648,692, entitled "Medical-Surgical Dressing For Burns And The Like", particularly that at column 1, lines 10-69.
The major problem with a burn wound is that the protective layer of skin is either missing or badly damaged at the wound site, so that the normal physiologic functions of the skin are absent or, at best, materially impaired. Two important physiologic functions of the skin are to serve as an antimicrobial barrier layer to prevent infection and to prevent the undue loss of body fluids, proteins and electrolytes. Once the skin can no longer adequately perform these functions, the body fluids, proteins and electrolytes are continuously lost and the invasion of harmful micro-organisms and other harmful agents into the body can proceed with predictable adverse results. For example, normal human skin has a water vapor phase transfer rate of about 2 mg./hr.-cm..sup.2, whereas the rate for burned skin can be 25 to 45 mg./hr.-cm..sup.2 or even higher during the first few hours after the burn. To alleviate these problems, the standard medical treatment for burns involves a combination of therapy and dressing to cover the burn site as soon as possible with a protective layer, the properties of which resemble the burned-away skin. While the use of topical and systemic antibacterial agents can reduce the extent of infection in a burn patient, coverage of the wound site with a skin-like dressing before the onset of infection remains a major factor in burn management.
At the present time, most of the burn dressings used by the medical profession are either human or animal skin. The more common of these dressings are generally referred to as "autografts", "allografts" (also sometimes called "homografts") and "xenografts" (also sometimes called "heterografts"). An autograft is a portion of the burn victim's own skin taken from an uninjured part of the body. The limitations of this dressing are apparent, especially in cases where the victim has suffered extensive dermal destruction. A homograft is skin taken from a cadaver. A xenograft is skin taken from a different species. Pigskin is the most commonly used xenograft. Autografts are generally preferred to homografts and pigskin xenografts.
The various human and animal skin dressings are expensive and are difficult to store for prolonged periods of time. Perhaps the most serious disadvantage of such materials, especially of homografts, is their limited availability. Over 100,000 burn victims were hospitalized in the United States alone every year. While not all such patients require a covering for their burn sites, it is estimated that 50 to 75% of those hospitalized would benefit from such a covering.
Recently, efforts have been made to develop synthetic burn dressings having physiologic properties similar to human skin which could be inexpensively prepared in large quantity and stored for long periods of time without degradation. See, for example, the polyurethane foam burn dressing described in U.S. Pat. No. 3,648,692. Kornberg and his coworkers have described a synthetic burn dressing composed of an ultra-thin (0.5 to 2 mils), pinhole-free silicone rubber membrane, to one surface of which a sheet of spun-bonded nylon, open-weave nylon or double-knit dacron is laminated. See "Ultra Thin Silicone Polymer Membrane: A New Synthetic Skin Substitute", Kornberg et al, Transactions of the American Society of Artificial Internal Organs, Vol. 18, pp. 39-44 (1972). The Kornberg et al dressing is impervious to bacteria, inert, non-antigenic, has a water vapor permeability similar to intact human skin, and is transparent and relatively inexpensive to produce.
Much of the success of the Kornberg et al dressing is attributed to the work of Nora E. Burns on techniques for mass-producing ultra-thin, pinhole-free silicone rubber membranes for use in membrane oxygenators. This work is described in "Production of Silicone Rubber Film for the Membrane Lung", N. Burns, Biomedical Engineering, Vol. 4, pp. 356-359 (1969). Briefly, Burns prepares her ultra-thin silicone rubber membranes by applying an extremely thick dispersion of silicone rubber to a moving horizontal surface and spreading the dispersion into a film of uniform thickness using an accurately ground doctoring blade. Major processing problems generally arise because of the extreme thickness of the silicone rubber membrane, which makes it very difficult to handle without damaging it, and the need for uniform thickness in order to obtain uniform properties throughout the membrane and establish satisfactory quality control.
Among the desired properties of a synthetic burn dressing are that it have water vapor phase transfer rates and anti-microbial barrier layer properties approximating those of human skin, that it adhere well to a wound, and that it preferably have voids in the surface applied to the wound for fibroblastic ingrowth of tissue and to serve as a debris reservoir for necrotic tissue and other debris from the wound, so that such materials are removed from the wound. The dressing should also be transparent or translucent, so the progress of the wound can be observed without removing the dressing. Two dimensional elasticity is another desirable property because it permits the dressing to expand and contract if applied to an elbow, knee or other body location where it is likely that the dressing will be flexed. The dressing should be drapable and readily conform to the shape of the body. It should possess a sufficiently high tensile and tear strength, so that the dressing can be handled and stretched without damage to the dressing. The uniformity throughout the dressing of such properties as the water vapor phase transfer rate and the anti-microbial barrier layer properties is also important, as is control of the characteristics of the dressing which affect these properties. Preferably, the dermal surface has controlled wicking characteristics to remove some but not all the fluid from the wound site, so as to maintain a medically acceptable, balanced fluid level on the wound site, and, of course, the dressing must be constructed from biologically innocuous and inert materials which are acceptable to the medical profession for use with human beings.
It is an object of this invention to provide improved synthetic polymeric wound dressings having the above properties and characteristics.
It is another object of this invention to provide processing techniques for preparing these improved synthetic dressings by which the properties and characteristics of the dressings can be varied in accordance with the contemplated end use of the dressings.
It is another object of this invention to provide improved silicone rubber burn dressings and processing techniques for preparing such dressings.
These and other objects of the invention will be apparent to those skilled in the art upon a consideration of the specification and attached drawings, taken in their entirety.